The present invention relates generally to an assembly for plugging a defective, leaking tube, and particularly to an assembly comprising explosive a plug, and a buffer, for sealing the ends of a defective tube in a heat exchanger.
Further, the present invention relates to an improved method of plugging a defective tube with the use of the above-said plugging assembly. The term "heat exchanger" used herein is to denote a nuclear-power steam generator, boiler, feed-water heater, cooler, condenser or chemical reactor, which all comprise numbers of closely spaced tubes of small diameters. In a heat exchanger, the primary fluid flowing inside the tube and the secondary fluid flowing outside the tube are of different temperatures from one another, and thus the heat exchange takes place between them.
Tubes in a heat exchanger can become defective for various reasons during a long-time use. If a tube becomes faulty during operation of the apparatus, the ends of that tube must be plugged immediately to prevent the primary and the secondary fluids from intermixing. The intermixing of the primary and the secondary fluids may cause many problems like leakage of radioactive material, decrease in heat efficiency and corrosion of the peripheral machinery.
Especially, in the case of a nuclear-power steam generator, if a tube gets damaged, the primary fluid including radioactive material contaminates the pure secondary fluid which rotates the turbine. Further, in the case of a boiler, the condenser or chemical reactor, if a tube gets faulty, salt water, undesirable chemicals or poisonous chemicals can leak out, thereby other facilities can be corroded and heat efficiency can decrease. Accordingly, the ends of a defective tube should be plugged immediately.
Quite a few methods have been developed and employed to plug the ends of a defective tube, such as, the general plugging by arc welding, plastic deformation plugging by mechanical or hydraulic enlargement of the tube and plugging by screws.
These conventional methods, however, can be employed only in the clean work area, and they cannot provide an immediate plugging. The plugging should be made promptly and flawlessly even under bad work environments, such as radioactive contamination, presence of poisonous material, high temperatures, or obstructed view.
The techniques relating to an explosive-activated plug and an improved method of plugging are disclosed in US Pat. No. 3,919,940, British Patent No. 1,380,964 and Japanese Patent laid-open No. sho 56-134,092. By these techniques, however, only permanent plugging is possible.
Furthermore, since they use a plug with quite an amount of explosive and a detonator inside, the welded sites of other, normal tubes can also be damaged due to the shock from the explosion, and the diameters of holes in tube sheet decrease after the explosion.